The previous condition didn't fail tests because it was
always false. The 'exists' property is not actually usable
when writing the interface file. It is merely a hint that
the interface existed in /proc/net/dev while parsing the
interfaces file and is otherwise actually unused here.

These two function could serve as a generic output sub for various CLI
utilities
* print_text_table prints an array of objects in a tabular fashion,
the formating is passed as an array containg hashes with titles, maximal
lengths and default values. This way we can stay extensible, by adding other
keys to the formatting options
* print_entry prints out a single entry, handling array-refs as properties